On the inner surface of a pipe, the outer surface of which is exposed, installed on the ground for transporting, for example, a fluid such as a gas or a liquid, or on the outer surface of a pipe, the outer surface of which is not exposed, installed under the ground, a thinned out portion caused by corrosion or the like may be present.
Presence of a thinned out portion as the defective portion as mentioned above on the inner surface or the outer surface of the Pipe ultimately results in such problems as production of a hole at the thinned out portion. It is therefore necessary to promptly detect the above-mentioned thinned out portion on the inner surface or the outer surface of the Pipe, and replace the pipe having such a thinned out portion with new one.
The following methods are known for detecting a thinned out portion as a defective portion on the inner surface of a pipe, the outer surface of which is exposed:
(1) Detecting method using radioactive rays:
This method comprises projecting radioactive rays such as X-ray or gamma-ray toward a pipe to be tested from the side of the outer surface thereof, measuring the amount of radioactive rays having passed through the pipe, and detecting a thinned out portion on the inner surface of the pipe by means of the amount of transmission of the radioactive rays.
(2) Detecting method using ultrasonic waves:
This method comprises transmitting ultrasonic waves toward a pipe to be tested from the side of the outer surface thereof, receiving reflected waves of the transmitted ultrasonic waves, measuring the time required up to receiving of the reflected waves, and detecting a thinned out portion on the inner surface of the pipe by means of the time required up to receiving of the reflected waves.
(3) Detecting method based on knocking:
This method comprises knocking a pipe to be tested from the side of the outer surface thereof with a hammer, for example, and detecting a thinned out portion on the Inner surface of the pipe by means of the thus produced sound.
(4) Detecting method based on cutting:
This method comprises cutting a pipe to be tested to permit an operator to directly observe the inner surface of the pipe, thereby detecting a thinned out portion on the inner surface of the pipe.
The following method is known for detecting a thinned out portion as a defective portion on the outer surface of a pipe, the outer surface of which is not exposed: (5) Detecting method using ultrasonic waves:
This method comprises transmitting ultrasonic waves toward a pipe to be tested from the side of the inner surface thereof, receiving reflected waves of the transmitted ultrasonic waves, measuring the time required up to receiving of the reflected waves, and detecting a thinned out portion on the outer surface of the pipe by means of the time required up to receiving of the reflected waves.
The detecting method using radioactive rays as mentioned in (1) above has the following problems:
(a) The detecting operation of a thinned out portion cannot be conducted unless the operator is qualified for handling radioactive rays. There is therefore a limitation in personnel.
(b) It is difficult to make a proper judgement on the result of detection, requiring high-level experience and technical knowledge.
(c) The detecting operation of a thinned out portion can be carried out only at a position closest to the outer surface of the pipe. When the pipe is installed at an elevated position apart from the ground, therefore, it is necessary to provide a scaffold for the detecting operation.
(d) The range of a single run of detection is narrow. It thus requires much time and labor for the detecting operation, leading to a low operating efficiency.
The detecting method using ultrasonic waves as mentioned in (2) and (5) above has the following problems:
(a) An error is often contained in the result of detection of a thinned out portion, thus preventing accurate detection.
(b) It is difficult to detect an extent and/or a depth of a thinned out portion.
(c) The range of a single run of detection, being only a point, is very narrow. It thus requires much time and labor for the detecting operation, leading to a low operating efficiency.
The detecting method based on knocking as mentioned in (3) above has the following problems:
(a) Determination of the presence of a thinned out portion requires high-level experience and technical knowledge, with furthermore much differences between individual operators, thus impairing accurate detection of the thinned out portion.
(b) It is impossible to detect an extent and/or a depth of a thinned out portion, and it is difficult to detect a small thinned out portion.
(c) The detecting operation of a thinned out portion can be carried out only at a position closest to the outer surface of the pipe. When the pipe is installed at an elevated position apart from the ground, therefore, it is necessary to provide a scaffold for the detecting operation.
(d) The detecting operation requires much time and labor, resulting in a low operating efficiency.
The detecting method based on cutting as mentioned in (4) above has the following problems:
(a) Use of the pipe must be interrupted for a while during cutting and detecting operations of a thinned out portion, and these cutting, detecting and restoring operations require much time and labor, resulting in a low operating efficiency.
(b) When the pipe is installed at an elevated position apart from the ground, it is necessary to provide a scaffold for the detecting operation.
Under such circumstances, there is a strong demand for the development of a method which permits certain, easy and efficient detection of the presence of a thinned out portion as a defective portion and an extent and/or a depth thereof, on the inner surface or the outer surface of a pipe, without the need for a special qualification, but a method provided with such properties has not as yet been proposed.